Analyte concentration determination in physiological samples is of ever increasing importance to today's society. Such assays find use in a variety of application settings, including clinical laboratory testing, home testing, etc., where the results of such testing play a prominent role in the diagnosis and management of a variety of disease conditions. Analytes of interest include glucose for diabetes management, cholesterol for monitoring cardiovascular conditions, and the like.
In response to this growing importance of analyte concentration determination, a variety of analyte concentration determination protocols and devices for both clinical and home testing have been developed and, specifically, a variety of analyte measurement devices and methods for enabling patients to test their own blood for the presence and concentration determination of a variety of different analytes are well known in the art. Of great interest and use in this area are optical based measurement devices and methods in which a sample is illuminated and reflected light therefrom is detected to obtain an analyte concentration. Of increasing interest in such optical based measurement protocols is the use of assay systems that employ test strips or cards and meters for reading these test strips. Typically, a physiological sample such as blood, blood derivatives, interstitial fluid, urine, etc. is introduced to a test strip to wet a particular testing or measurement area of the test strip. The sample reacts with certain reagents or components associated with the testing area to produce a color change in those areas where the test strip has been wetted by the sample. Reflected light detected from this testing area is what is used to obtain an analyte concentration, as mentioned above, by relating the amount of reflected light to analyte concentration.
A characteristic of devices and methods that provide for analyte concentration determination using a measured reflectance value is that sample size and the uniform or even distribution thereof can have an impact on the final measurement, where a sample size that is too small or a sample that is not uniformly applied can cause erroneous or inaccurate results. Specifically, if an insufficient volume of sample is applied to the test strip and/or sample is not uniformly applied, only a portion of the testing area is wetted by the sample while other portions of the testing area are not wetted. In conventional optical based measurement devices and methods, light is detected from the entire testing area, including those portions that are not wetted by sample. However, the use of light detected from non-wetted portions of the testing area can cause the determination of analyte concentration to be erroneous or inaccurate.
Attempts at solving the above described problems of insufficient and/or non-uniformly applied sample have not been wholly adequate. In the simplest process, it is incumbent upon the user to visually monitor whether sufficient sample has been applied and whether the testing area has been uniformly wetted. However, such visual monitoring is not very reliable, especially for persons with diabetes who typically have impaired vision.
In another attempt to solve the problems described above, EPB0087466 describes an apparatus that estimates whether the amount of sample is sufficient on the basis of absorption of water in the infrared region of the electromagnetic spectrum. However, such an apparatus requires a means for quantitative analysis and an infrared transducer and receiver and is therefore disadvantageous for use as a portable system such as for home glucose testing by diabetics. Furthermore, in using the apparatus disclosed in EPB0087466, instances where sample is non-uniformly applied can not be easily identified.
U.S. Pat. Nos. 5,889,585 and 6,055,060 also attempt to solve the problem described above by comparing values obtained from two different test strip sites to each other, where a certain magnitude of deviation indicates that the measuring field is not uniformly wetted. When non-uniformity is indicated, the user is prompted to apply more sample or, in certain instances, prompted that too much time has elapsed and a new test must be commenced. That is, neither the '585 patent nor the '060 patent provides for analyte concentration determination using the small sample provided and/or the unevenly wetted measurement area and instead requires the user to apply more sample to the test strip or begin a new test. Neither of these options is wholly satisfactory.
In the instance where the user is prompted to apply more sample, the user either has to attempt to squeeze the initial site of the needle stick in order to try to “milk” or massage more blood out of that site or the user must pierce their skin once again at another site. Because blood clots quickly, by the time the user is prompted to apply more blood, it is not uncommon that another site must be pierced with a needle. The piercing procedure for obtaining sample can be painful, where it will be obvious that the pain is compounded when the skin needs be pierced multiple times in order to obtain the requisite sample volume to conduct the test. Due to this pain, it is not uncommon for individuals who require frequent monitoring of an analyte to simply avoid monitoring the analyte of interest all together. With diabetics, for example, the failure to measure their glucose level on a prescribed basis results in a lack of information necessary to properly control the level of glucose. Uncontrolled glucose levels can be very dangerous and even life threatening.
In the instance where a new, second test must be commenced, a new test strip is required for the second test. As such, the test strip used for the first, uncompleted test is discarded in place of a new test strip for use with the second test, resulting in the use of two test strips, rather than one, for a single analyte concentration determination. This increases the already high cost of test strip-based analyte concentration determination.
As such, there is continued interest in the development of new devices and methods for analyte concentration determination that provide accurate analyte concentrations in the cases where small sample volumes are applied to a test strip measurement area and/or the measurement area of the test strip is not evenly or uniformly wetted by sample. Of particular interest would be the development of such devices and methods which are easy to use, particularly for visually impaired individuals, involve minimal pain and are portable.